Reproductive behaviour in mammals is directly influenced by both internal and external factors such as photoperiod, temperature, nutrition, since these change each season, some animals become seasonal breeders. The bitch presents estrus once or twice a year but it is not clear what influences this behaviour. Several reports of research done in Mexico City show two peaks of apparently increased reproductive activity, however photoperiod did not fully explain the effect. In the United States, Tedor in 1978 found that in the states where temperature was more constant throughout the year the number of whelpings was higher, in comparison with those where the temperature changes were more drastic, also that the highest number of births was in July, which is the warmest month in the United States. Therefore the purpose of the present work was to study the effect of temperature in the reproductive behaviour of bitches in Mexico City, where there is a big problem of stray dog overpopulation. A total of 10305 birth certificates from the Mexican Kennel Club were analyzed, they were from the period of January 1996 to December 2003. They were grouped by breed and month of estrus into 3 month periods (1st. Period December-February, 2nd period March-May, 3rd period June-August and 4th period September-November). For statistical analysis the tests used were Kruskal-Wallis and Tukey. Results did not show any variation in the month-breed effect in the 8 evaluated breeds. However for the period-breed effect, and only in the 4th period the Yorkshire breed showed significant difference (P<0.05 ) from the other 7. In conclusion reproductive behaviour appears constant through the 8 years of study, which would agree with the work of Tedor since the temperature in Mexico City is very constant through the year with no drastic changes, therefore in Mexico City the bitch is a non seasonal monoestrous animal.